A known pressure sensor, as shown in U.S. Pat. No. 4,716,492, the subject matter of which is included herein by this reference, comprises a capacitive pressure sensing element having a thin, relatively flexible ceramic diaphragm mounted in closely spaced, sealed, overlying relation to a rigid ceramic substrate. Metal coatings are deposited on respective opposing surfaces of the diaphragm and substrate to serve as source and detect capacitor plates which are arranged in predetermined closely spaced relation to each other to form a capacitor. Electrically conductive traces extend from the capacitor plates out to pins received in bores formed through the substrate located between the capacitor plates and the outer periphery of the diaphragm and substrate which are connected to an electronic conditioning module attached to the transducer. The diaphragm flexes in response to pressure and causes the source and detect plates to move closer together thereby increasing the capacitance between the plates which is measured by the electronic conditioning module. An annular guard ring of electrically conductive material is printed on the substrate around the detect plate and electrically held at the same voltage as the detect plate. This ring serves as a guard to reduce the electrical field intensity between the source and detect plates at the edges of the detect plate. These fringe electric fields are undesirable because they cause a non-linear pressure transducer output. The electronic conditioning module is designed to measure the capacitance between the source and detect plates only and is insensitive to capacitance between the source plate and the guard, between the detect plate and the guard or between either the source plate or the detect plate and the housing of the sensor.
When used with polar or conductive fluids it has been found that the transducer output shifts by up to 1% full span or more. In view of the fact that the pressure transducers are used to monitor the pressure of many fluids including those which are polar or conductive, such as water, this error is undesirable.
One proposed solution is to place a thin discrete metal shield on the diaphragm connected to the transducer housing through brass wool or similar electrically conductive material. The conductive shield covering the diaphragm and connected to the housing would act as a guard for the entire transducer, that is, the electric fields would not pass through the conductive shield and, therefore, could not be affected by material on the opposite side of the shield. However, this approach is unsatisfactory for several reasons including the possibility of pieces of the wool deteriorating and contaminating the fluid, the effect of pressure from the compliant wool on the transducer output, the durability of the metal shield and questions of compatibility with various working fluids, possible hysteresis due to the metal shield and the question of long term durability of the electrical contact between the shield and the housing.
Another proposed solution is the use of a metal shield printed on the surface of the diaphragm using, for example, the same material, e.g., gold, which is used for the electrically conductive capacitor plates and traces. The printed shield would be connected to the metallic housing using a compliant mechanism such as a washer and a wave spring. However, this approach involves the addition of components which add to the cost of the sensor as well as causing problems relating to pinching and damaging an adjacent O-ring used as a fluid seal.
In copending application Ser. No. 09/067,162, assigned to the assignee of the present invention, an improved guard plate arrangement is shown and described comprising a source guard plate which surrounds the source plate, source trace and source pin aperture on the diaphragm and a detect guard plate which surrounds the detect plate, detect trace and detect pin aperture on the substrate in order to prevent pressure transducer output errors due to the presence of polar or conductive fluids. However, while this approach is effective in many applications, in certain other applications, particularly low pressure applications in which the diaphragm is relatively thin, output error continues to be higher than desired.
Yet another approach, which involves the provision of a conductive shield, e.g., gold layer, on the surface of the diaphragm as described above, employs a conductive wire which is bonded to the diaphragm and connected to a ground trace on a flexible circuit which mounts the electronic conditioning module within the housing. The ground trace on the flexible circuit is in turn connected to the housing. Although this approach is effective in shorting the parasitic coupling field lines to ground through the housing for diaphragms of any thickness, each unit requires hand assembly making it costly as well as fraught with risk of damage during handling and unsuitable for production in large quantities. It would be desirable to provide a sensor in which the output error is minimized or eliminated without appreciably impacting the cost of the sensor.